The life of a light bulb is limited by two factors-the high temperature of the filament during normal operation and the surge current which flows through the filament when the bulb is switched on by a standard switch, typically single pole single throw. In fact, it is commonly observed that light bulbs burn out at the instant they are switched on; this indicates the harm done by the initial surge current. This current heats the filament rapidly, and weakens it by "thermal shock". Also, in sensitive electronic systems, the surge current can disrupt other electrical components powered by the same power supply or sharing the same ground plain.
If the filament of the light bulb system survives this rapid heating, its impedance will increase, and it will operate at the steady state, rated current.
The surge current can be decreased if the filament is always kept warm with a conventional "keep-alive" resistor providing a continual and substantial current to the bulb. See Petschauer Pat. No. 3,388,293.
A second conventional way to decrease the initial surge current is to insert a current limiter between the power source and the bulb. Note that this current limiting circuitry passes not only the initial current but also the steady state, rated current.
A third known circuit to limit the surge current is shown in Stevens, Pat. No. 3,612,945 and Irie, Pat. No. 3,742,295. These circuits provide gradual turn-on current to the bulb once the switch is situated in the on position. They utilize a resistor to gradually charge a capacitor when the switch is turned-on. Until the capacitor charges to a threshold level, a limited current is delivered to the bulb through another resistor, and once the threshold capacitor voltage is reached, a triac is activated and delivers the operating current to the bulb. Note that there is always some electrical component between the power source and the bulb during the bulb's operation.
Another conventional device to limit initial surge current is a rheostat which utilizes a variable, transformer to control the voltage across the filament manually. The voltage is gradually increased by manually turning the rheostat dial after the filament is warmed by the lesser voltage. Also, there are "dimmer" switches which utilize a rheostat or a variable, series inductance and are manipulated with a switch like lever instead of an obvious dial.
Also, Lawson Pat. No. 3,309,544 discloses a three position switch having off, half power, and full power switch positions.
The parent application to this one, Ser. No. 583,350, provides a warm-up current to a load when the switch is between the off and on positions, and afterwards provides a direct short to the load when the switch is on. But, in the event that the switch is purposely held in transition, the parent invention will continue to deliver the warm-up current and dissipate power internally.